If I were to say "2-3 weeks" it seems clear that weeks should be plural. Are there any cases where a number range would not be plural (e.g. 0-1 week or 0-1 weeks)? Would it matter if the usage was in a legend versus being written in a sentence?
It is certainly not always plural, but that does not mean I know of a "rule" for the form it should take.
Fractional numbers greater than zero but less than one are singular. If your range were, for example, one-tenth to one-half of a mile, then "mile" must be singular.
By convention, when we express ranges we almost always start the range with the smaller number and end with the larger number. If we only examine that convention, it is likely we will find that the larger number calls for the plural form.
Consider expressing a range with the smaller number last, however, and some interesting things can happen.
He had at most four, but as few as one, book in his bag.
I cannot cite a "rule", but I feel that "books" would be odd and distracting in the above sentence.
Therefore, my intuition is that the last quantity in the range controls whether to use the singular or plural form.